TEMPORAL BEHAVIOUR OF THE OPTOGALVANIC SIGNAL IN A HOLLOW CATHODE LAMP

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TEMPORAL BEHAVIOUR OF THE OPTOGALVANIC SIGNAL IN A HOLLOW

CATHODE LAMP

M. Broglia, F. Catoni, P. Zampetti

To cite this version:

M. Broglia, F. Catoni, P. Zampetti. TEMPORAL BEHAVIOUR OF THE OPTOGALVANIC SIG-

NAL IN A HOLLOW CATHODE LAMP. Journal de Physique Colloques, 1983, 44 (C7), pp.C7-479-

C7-487. �10.1051/jphyscol:1983747�. �jpa-00223304�

TEMPORAL BEHAVIOUR OF T H E OPTOGALVANIC S I G N A L IN A HOLLOW CATHODE LAMP

M. B r o g l i a , F. C a t o n i and P . Z a m p e t t i

ENEA, Centro Ricerche Energia Casaccia, COMB, Divisione Ingegneria Sperimentale Arricchimento,

P.O.

Box 2400, 00100 Roma A.D., I t a l y

RQsurnQ - Le s i g n a l o p t o g a l v a n i q u e i n d u i t p a r un l a s e r p u l s g d a n s une lampe c a t h o d e c r e u s e d ' u r a n i u m montre, d a n s de nombreux c a s e x p g r i m e n t a u x . deux composantes t e m p o r e l l e s : une cornposante l e n t e c a r a c t g r i s e e p a r l e temps de r e t a b l i s s e m e n t du regime permanent d a n s l a dgcharge e t une a u t r e beaucoup p l u s i n t e n s e e t r a p i d e . Une Q l e c t r o n i q u e r a p i d e nous a perrnis de rnesurer l a l a r g e u r de c e d e r n i e r s i g n a l q u i e s t l a rnSrne que c e l l e de l ' i m p u l s i o n l a s e r . C e c i p e u t - S t r e dG

2

une i o n i s a t i o n d i r e c t e p e n d a n t l ' i m p u l s i o n l a s e r . Un g r a n d nornbre d161Qrnents e x p e r i m e n t a u x c o n f i r m e c e t t e h y p o t h e s e .

A b s t r a c t

-

The o p t o g a l v a n i c s i g n a l i n d u c e d by a p u l s e d dye l a s e r i n a n u r a - nium h o l l o w c a t h o d e lamp h a s been d e t e c t e d . I n many e x p e r i m e n t a l c a s e s t h e s i g n a l s h a p e shows two t e m p o r a l components: a slow one h a v i n g t i m e s t y p i c a l o f t h e d i s c h a r g e r e l a x a t i o n t o t h e s t e a d y s t a t e , and a f a s t more i n t e n s e one.

F a s t e l e c t r o n i c c i r c u i t r y a l l o w e d t o measure t h e w i d t h o f t h e f a s t component, t h a t t u r n s o u t t o b e e q u a l t o t h e l a s e r p u l s e d u r a t i o n . T h i s may be i n t e r p r e t e d a s a d i r e c t i o n i z a t i o n d u r i n g t h e l a s e r p u l s e . Much e x p e r i m e n t a l e v i d e n c e s u p p o r t s t h i s hypothesis.

INTRODUCTION

D e t e c t i o n o f t h e o p t o g a l v a n i c ( O G ) e f f e c t on p u l s e d l a s e r e x c i t a t i o n , a l l o w s t o s t u - dy t h e t r a n s i e n t b e h a v i o u r o f t h e p r o c e s s .

G e n e r a l l y , dynamic o p t o g a l v a n i c measurements / l / show c h a r a c t e r i s t i c t i m e s i n t h e

rnL

c r o s e c o n d r a n g e . These t i m e s a r e g i v e n by d i f f e r e n t phenomena w h i c h , a f t e r r e s o n a n t e x c i t a t i o n o f t h e a t o m i c s y s t e m , g i v e r i s e t o a d i s c h a r g e impedance v a r i a t i o n t h r o u g h a n i n d i r e c t c h a r g e p r o d u c t i o n , f o l l o w e d by c h a r g e c o l l e c t i o n and s y s t e m r e l a x a t i o n t o t h e e q u i l i b r i u m s t a t e / 2 / . Of c o u r s e c a r e must be t a k e n i n o r d e r t o a v o i d t h a t t h e l e c t r o n i c c i r c u i t r y r e s p o n s e a f f e c t s t h e t y p i c a l t i m e s o f t h e e f f e c t .

During a n OG e f f e c t s t u d y c a r r i e d o u t o n . t h e 5915

8

uranium t r a n s i t i o n / 3 / , we n o t e d t h a t on c h a n g i n g e i t h e r t h e t u n i n g o r t h e power o f t h e l a s e r , t h e s i g n a l s h a p e was a l s o changed. I n p r a c t i c e , a f a s t e r and more i n t e n s e s i g n a l a r o s e t o g e t h e r w i t h t h e PS one. S i g n a l d e t e c t i o n by a f a s t a m p l i f i e r a l l o w e d u s t o measure t h e f a s t s i g n a l d u r a t i o n and t o f i n d it was t h e same a s t h e l a s e r p u l s e . E x p e r i m e n t a l e v i d e n c e o b t a L ned u s i n g d i f f e r e n t e x c i t a t i o n schemes, a l l o w s t o r e l a t e t h i s s i g n a l t o a p h o t o i o n i - z a t i o n p r o c e s s o c c u r r i n g d u r i n g t h e a t o m - r a d i a t i o n i n t e r a c t i o n .

EXPERIMENTAL SETUP

The e x p e r i m e n t a l s e t u p i s shown i n F i g u r e 1. We u s e two dye l a s e r pumped by t h e s e - cond o r t h i r d harmonic o f a Nd-YAG l a s e r . The dye l a s e r s have a 1 0 GHz bandwidth w i t h o u t i n t r a c a v i t y e t a l o n and 1 GHz w i t h i n t r a c a v i t y e t a l o n . I n t h e narrowband s e t u p we can change t h e w a v e l e n g t h by a c a v i t y p r e s s u r e c o n t r o l s y s t e m .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983747

JOURNAL DE PHYSIQUE

M O N O C H R O M A T O R

/

P D P 1 1 / 3 6

1

H C L

@ i°KQ

F i g . 1

-

E x p e r i m e n t a l s e t u p

The two l a s e r beams a r e a l i g n e d t o g e t h e r i n t o a U/Ne commercial h o l l o w c a t h o d e lamp (Westinghouse E l e c t r i c C o r p . ) . A m i r r o r , d r i l l e d i n t h e c e n t r e t o l e t the.bearn t h r o - g h , a l l o w s c o l l e c t i o n o f t h e l a s e r i n d u c e d f l u o r e s c e n c e on t h e e n t r a n c e s l i t o f a dog b l e rnonochromator. The w a v e l e n g t h r e s o l v e d s i g n a l i s a c q u i r e d by a photon-counting s y s t e m . The peak power and t h e t e m p o r a l s e q u e n c e o f . t h e l a s e r p u l s e s a r e measured by a vacuum p h o t o d i o d e . We used c i r c u i t ( a ) f o r m o n i t o r i n g t h e s l o w p a r t o f t h e o p t o g a l v a n i c s i g n a l and c i r c u i t ( b ) f o r t i m e r e s o l v e d measurements. The s i g n a l was a c c u i r e d and p r o c e s s e d u s i n g e i t h e r a n o s c i l l o s c o p e o r a Boxcar a v e r a g e r o r a T r a n s i e n t D i g i - t i z e r .

FAST AND SLOW SIGNALS

F i g u r e 2 shows some OG s i g n a l s o b t a i n e d by l a s e r e x c i t a t i o n around 5915

8 ,

a t d i f f e r e n t power and d e t u n i n g v a l u e s , employing c i r c u i t ( a ) o f F i g u r e 1. I n t h e f i r s t p i c - t u r e t h e l a s e r i s t u n e d o n t h e 0-16900.4-cm -1 uranium transition and t h e power i s h i g h enough t o s a t u r a t e t h e a b s o r p t i o n / 3 / ; i t c a n be s e e n i n t h e f o l l o w i n g p l c t u r e s t h a t , on i n c r e a s i n g e i t h e r t h e f r e q u e n c y o r t h e power, t h e s i g n a l s h a p e c h a n g e s t o - ward a f a s t e r b e h a v i o u r .

I t s h o u l d be n o t e d t h a t i t i s p o s s i b l e t o e x c i t e t h e a t o m i c uranium on t h e 33801.0

-1

-1

cm l e v e l by a two-photon p r o c e s s , on i n c r e a s i n g t h e f r e q u e n c y by - 0 . 1 cm o n l y . A c l o s e r e l a t i o n s h i p between t h e change i n t h e s i g n a l t e m p o r a l b e h a v i o u r and t h e h i - g h e r l e v e l e x c i t a t i o n i s t h e r e f o r e s u g g e s t e d . I n o r d e r t o c h e c k t h i s , t h e l a s e r wave l e n g t h was s c a n n e d a r o u n d t h e r e s o n a n t v a l u e and t h e s i g n a l was d e t e c t e d by a b o x c a r a v e r a g e r a t t h e two t i m e p o s i t i o n s shown i n F i g u r e 2 . A t t h e same t i m e t h e l a s e r i n - duced f l u o r e s c e n c e s i g n a l a t d i f f e r e n t f r e q u e n c i e s was r e c o r d e d .

cess occurrence. In our case the 760 nrn fluorescence corresponds to 16900.4 cm -I excitation and the 375 nm fluorescence corresponds to 33801.0 cm

-1

two-photon exci tation. In the measurements described a lamp working in particular low current con- ditions was used (300 p A , 350

V).

In these conditions the continuous emission is ve- ry low and it is possible to carry out the induced fluorescence measurements without any background.

The results are shown in Figure 3. The two peaks in the 375 nrn fluorescence corres- pond to progressive tunlng on the two-photon and two-step transition towards the

-1 -1

33801.0 cm level: in the latter case the 16900.4 cm level is populated by off- resonance excitation. On comparing the fluorescence and OG signals (slow and fast) one can notice that, in the longest wavelength range only the slow OG signal is ob- served, corresponding to the 760 nm fluorescence. On decreasing the wavelength, the fast signal shows up as well on the two-photon transition; the second peak, on the two-step transition, is not resolved from a larger one corresponding to a wavelength excitation range in which no fluorescence signal is present.

JOURNAL DE

PHYSIQUE

1 6 9 0 1 . 0 1 6 9 0 0 . 7 1 6 9 0 0 . 4 1 6 9 0 0 . 1

-

^{5 (cm-')}

Fig. 3 - Laser scanning around the 0-16900 cm -1 uranium transition with simultane- ous detection of OG and fluorescence signals.

(a) Slow OG signal; (b) Fast OG signal; (c) Fluorescence signal at 760 nm;

( d ) Fluorescence signal at 375 nm.

This large resonance is probably due to a three-photon excitation up to an autoioni- zing state.

We may conclude that the slow part of the OG signal corresponds to one-photon excita tion on 16900.4 cm

-1

while the fast part corresponds to multiphoton excitation on higher-lying levels.

FAST SIGNAL DETECTION

The measures we described in the previous paragraph show that the fast and the slow part of the OG signal have a different behaviour, so that we can in fact consider the superposition of two signals with different characteristic times. This behaviour is noticed in many other uranium transitions and in particular in all the ones with final level above half the ionization limit. So far, however, we know nothing about the duration of the fast signal, which could be masked by the response times of the circuitry we used.

tegration time of the order of w 1 ns. On the other side, the 5 0 0 impedance does not allow an efficient detection of the slow part of the signal which is less inten- se than the fast one, and, it was therefore often necessary to take the output from the lamp anode; thus two outputs are available.

This circuitry allows detection of a signal as fast as that shown in Figure 4(a).

The signal has been acquired by a Transient Digitizer and PDP 11/34 mi~icomputer, averaging 100 signals; we also subtracted the background generated by the electrorna- gnetic induction of the pump laser discharge already minimized by shielding the de- tection system. The same signal is shown in Figure 4(b) on a one hundred times nar- rower time scale. A tail appears that is much slower and less intense than the fast

0

peak. The signal has been obtained exciting the atomic system on the 3894 A uranium line, aligning the laser beam into t!ie cathode of the lamp in working conditions of 20 mA. The laser power is 20 kw/cm2 on a bandwidth of 10 GHz.

0

Fig.

4 - Time resolved OG signal for the 3894 A uranium transition.

(a) 100 ns range; (b) 10

p d

range.

As we see in Figure 4(a) and also for the other fast signals that are obtained by different excitation schemes, the signal has the same duration as the laser pulse

(

6 ns

_{) .}

This experimental result suggests the occurrence of a direct

ionization

process du- ring the laser pulse. In the case shown in Figure 2 one can suppose, for instance, that after 0-33801.0 cm -1 two-step or two-photon excitation, another photon carries out the ionization

(

IN50000 cm -1

) ,

while, in the case of Figure

4

the direct photo ionization can be accomplished by a two-step process.

CORRESPONDENCE BETWEEN THE FAST SIGNAL AND THE PHOTOIONIZATION PROCESS

To make sure that the fast signal arises from a photoionization process a two-step

excitationemployingtwo dye lasers has been realized: the first laser excites urani-

um up to the 25672 cm

-1

level

(

3894 8

)

and the second one up to an autoionizing

state around 50440 cm -1 . The second laser is optically delayed with respect to the

first one. In Figure

5

the OG signals are reported as well as the vacuum photodiode

signal.

JOURNAL D€ PHYSIQUE

Fig. 5 - Two-step photoionization process: vacuum photodiode and optogalvanic signals (a) Laser pulses sequence (photodiode signal)

( b )

Fast OG signal at high power laser

1

(c) Fast OG signal at low power laser

1

(d) Slow OG signal at low power laser 1

(e) Fast OG signal at low power laser lpluslaser

2

In Figure 5(,a) the photodiode signal shows the time delay between the two laser pul ses ( ~ 1 7 ns). Figure 5(b) shows the OG signal when only the first laser, working at high Gower, photoionizes the uranium. Then the power is decreased 100 times so that the fast signal disappears and only the slow one is detectable (Figure 5(c) and 5(d)) 4t last (Figure 5(e)) the second laser is switched on too and the fast signal appe- ars syncronous with this pulse. Of course the second laser alone does not produce any OG effect.

Finally we checked that when the excitation wavelength and power cannot produce

photoionization only the slow optogalvanic signal occurs. We have examined some of

the fast signal does not arise at any laser power we used.

T r a n s i t i o n W a v e l e n ~ r h I n t e n s i t y F a s t S i g n a l

(cm-') ( K , 1 4 1 o c c u r r e n c e

0 - 2 5 8 2 5 . 6 3871.0 189.0 Y E S

0-25672.5 3894.1 139.0 Y E S

0 - 2 5 3 4 9 . 0 3943.8 153.0 Y E S

Table I - Fast signal occurrence for some of the most intense uranium transitions.

SLOW AND FAST SIGNAL BEHAVIOUR AS A FUNCTION OF THE LASER POWER AND BEAM POSITION The fast and slow signal behaviour versus l2ser power was obtained exciting the ato- mic system along the cathode axis, at

3894

A, with uniform laser beam of

2

mm dia- meter ( d h a l f the internal cathode diameter) and a lamp current of

20

mA.

The results are shown in Figure

6.

The signals have been detected by a boxcar avera- ger using both the fast and the slow output of our circuitry.

L a s e r P o w e r (kw/cm2)

Fig.

6 - Slow and fast signal behaviour as a functio,? of the laser power.

C7-486 JOURNAL DE PHYSIQUE

According t o t h e o t h e r e x p e r i m e n t a l e v i d e n c e a t low power o n l y t h e OG s i g n a l , c h a r a c t e r i z e d by p s t i m e s , a r i s e s . Although t h e OG s i g n a l t e n d s t o s a t u r a t e w i t h i n c r e a s i n g power, t h e growth o f t h e i o n i z a t i o n f a s t s i g n a l masks s u c h s a t u r a t i o n w i t h a s t r o n g t a i l t h a t e x t e n d s i n t o t h e p s r a n g e .

Using t h e same a p p a r a t u s we have i n v e s t i g a t e d how t h e s l o w and f a s t s l g n e l s depend on t h e beam p o s i t i o n i n t h e c a t h o d e . We s e t t h e l a s e r power a t -7 kw/cm2 t o e n s u r e a n

i n

t e n s e f a s t s i g n a l and s h i f t e d t h e lamp p a r a l l e l t o t h e beam. Thc r e s u l t s a r e shown i n F i g u r e 7.

B e a m P o s i t i o n (mm)

F i g . 7 - Slow and f a s t s i g n a l b e h a v i o u r s c a n n i n g t h e beam p o s i t i o n a c r o s s a d i a m e t e r o f t h e i n t e r n a l c a t h o d e s e c t i o n . ( a ) Slow s i g n a l ; ( b ) F a s t s i g n a l .

We c a n n o t i c e t h a t t h e f a s t s i g n a l i n c r e a s e s n e a r t h e w a l l s and t h a t t h e s l o w p a r t i n - c r e a s e s a l s o i n t h e c a t h o d e c e n t e c where t h e f a s t s i g n a l k e e p s c o n s t a n t . We have ex- p e r i m e n t a l l y proved t h a t t h e f a s t s i g n a l i n c r e a s e i s n o t c a u s e d by t u n i n g i n d i p e n d e n t phenomena c o n n e c t e d w i t h t h e l a s e r s t r i k i n g t h e c a t h o d e s u r f a c e . I n f a c t we have no o f f - r e s o n a n c e s i g n a l f o r any beam p o s i t i o n . We c a n n o t g i v e a t p r e s e n t a n e x h a u s t i v e e x p l a n a t i o n o f t h e s e r e s u l t s f o r which f u r t h e r e x p e r i m e n t a l i n v e s t i g a t i o n i s n e c e s s a - r y .

to one and two-photon absorption. Furthermore, the ionization signal occurence enhan- ces the OG signal: the transitions above half the ionization limit prove this state- ment. Finally, the direct ionization during the laser pulse provides a lot of charges in a short time interval and might be very useful to get information about the collec- tion times.

REFERENCES

/l/ MIRON E., SMILANSKI I., LIRAN

J., LAVI S. and EREZ G., IEEE J.Quant.Electr., QE-15 (1979) 194

/2/

ERE2 G., LAVI S. and MIRON E., IEEE J.Quant.Electr.,

QE-15

(1979) 1328

/ 3 /

BROGLIA M., CATONI F. and ZAh!PETTI P., EGAS Conference (Liegi 1982)

/4/ PALMER B.A., KELLER R.A. and ENGLEMAN R.Jr., LA-8251-MS (1980)